

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode* left, TreeNode* right)
        : val(x), left(left), right(right) {}
};


TreeNode* invertTree(TreeNode* root) {
    if(root == nullptr) return root;
    auto left = invertTree(root->left);
    auto right = invertTree(root->right);
    root->left = right;
    root->right = left;
    return root;
}